Head right and down the corridor. Open the green door east. Go down the corridor to the very north. If you go left down the stairs, you notice that yellow key is inaccessible right now. So go right (east) and go upstairs. Pickup the rocket launcher. This is enough to do in order to get access to the yellow key. Track back to the corridor and use the western stairs. Pickup the yellow key.

Backtrack to the beginning of the level via the green door and go to the outside area in the middle of the level. Use the alcove east and open the yellow door. Get the blue key and backtrack outside. Use now the blue exit door north and press the skull for the exit.

Enter the green door in the east, turn right and open the wooden wall with face on it. You could also notice the candle on the floor. Enter teleporter in the closet behind this wall. In the garret, take BFG9000 and bulk cell. (sector 75)